Does plant diversity affect the water balance of established grassland systems

Evidence from experimental grasslands indicated that plant biodiversity modifies the water cycle, but it is unclear if this is also true for established land-use systems. Therefore, we investigated how actual evapotranspiration (ETa), downward flux (DF), and upward flux (UF) in soil are related with land use and plant diversity in agriculturally managed grassland. In three Central European regions (Biodiversity Exploratories), we studied 29 grassland plots (50x50m; 9-11 plots per region) covering the land-use classes pasture, mown pasture, and meadow, in at least triplicate per region. From 2010 to 2015, we measured soil moisture, meteorological conditions, plant species richness, cover, and number of species in the functional groups of grasses, herbs, and legumes, aboveground biomass and root biomass on each plot. Annual ETa, DF, and UF were calculated for two soil layers with a soil water balance model and statistically analysed for land-use and biodiversity effects with analysis of variance. Water fluxes were not significantly affected by land-use class. UF did not vary between plots with different species richness and plant functional group composition. DF from topsoil increased with increasing number of grass species. ETa from topsoil decreased with increasing species richness and with the number of herb or legume species, whereas ETa from subsoil increased. Our results demonstrate that plant diversity influences the soil depth partitioning of water use, but the complex drivers of this relationship in agriculturally managed grassland still need to be disentangled.